The basic body plan of all vertebrate embryos consists of three concentric cylinders of tissue, the innermost endoderm, destined to form the gut lining; the superficial ectoderm, which will give rise to the neural tissue and epidermis; and the middle mesoderm layer which becomes the muscle, bone, cartilage, heart, connective tissue and the limbs of the body. The long-term goal of this study is to understand the mechanism by which this basic body plan arises, using the Xenopus/aevis embryo as the model of choice. The maternally supplied transcription factor VegT plays a pivotal role in this process, since embryos lacking VegT are unable to gastrulate, and do not form endodermal or most mesodermal derivatives. VegT activates the expression of a number of transcription factors and signaling molecules at the late blastula stage. This work focusses on the question of how VegT controls mesoderm formation. The hypothesis is that factors downstream of VegT, together with other matemal regulators, pattem the mesodermal germ layer. Aim 1 and 2 study the specific roles in this process of a subset of VegT-target genes, the transcription factors Mixer and GATA 5, and the signaling molecules of the nodal and FGF classes, using loss of function analysis. Aim 3 examines the role of the enzyme PACE 4 in regulating Xnr signaling, and Aim 4 analyses the function of another matemally expressed transcription factor, CREB in mesoderm formation. Given the conservation of the nodal and FGFsignaling pathways across the phyla, the findings from the proposed studies will be applicable to higher vertebrates. Understanding the chain of events whereby pluripotent embryonic cells become restricted to specific developmental programs is a central goal of modern developmental biology, since many human congenital diseases arise as a result of the failure of this process, or because of the loss of the differentiated cell population. This work provides important information on the steps that convert pluripotent cells to differentiated states, and will assist in the goal of making therapeutic intervention a possibility.